1. Technical Field
The present invention relates to a structure of a wire bonding apparatus and a bonding method employed by such a wire bonding apparatus.
2. Description of the Related Art
In an assembling process of a semiconductor such as an IC, a wire bonding apparatus that typically connects a pad of a semiconductor chip with a lead of a lead frame using a wire is used. According to the wire bonding apparatus, bonding to the pad is performed using a capillary by causing a wire to extend from a tip end of the capillary and forming the extended wire into an initial ball using such as an electronic flame off. Subsequently, the capillary is moved upward and toward the lead while feeding the wire in a looping operation, and then bonding the wire to a second bonding point is performed using the capillary. Upon completion of the bonding the wire to the lead, the capillary is moved upward while feeding the wire from the tip end of the capillary, and then a clamper is closed. Then, the capillary and the clamper are moved further upward, and whereby the wire is cut in a state in which a tail wire of a predetermined length is extending from the tip end of the capillary. Subsequently, the capillary is moved to a position of the electronic flame off, the tail wire rolls up into an initial ball, and then bonding the wire to the next first bonding point is performed. Pads and leads are sequentially connected by repeating bonding in this manner.
The wire can be cut by holding the wire with the clamper and moving the wire upward after bonding to the lead, because the wire can be easily cut at an end portion of the wire bonded to the lead as this portion is crushed with the capillary when bonded and therefore the cross-sectional area is smaller than in other portions of the wire.
However, the cross-sectional area of the end portion of the wire bonded to the lead is often not sufficiently small depending on the bonding condition. In such a case, holding the wire with the clamper and moving the wire upward cannot cut the wire easily, and the wire is cut only after a large tensile force is applied to the wire. This causes the wire to flip up due to a reaction force of the large tensile force applied to the wire when cutting, and causes the wire and the tail wire under the clamper to bend in an S shape. The bend in the tail wire often results in defective ball formation that is carried out by such as electric discharges. Even if a ball is formed in a good condition, the wire in the capillary or between the capillary and the clamper still includes the bend. Accordingly, performing the bonding using such a bent wire adversely causes the wire connecting the pad and the lead to include an S-shaped portion, and possibly results in a problem that two adjacent wires are brought into contact with each other.
Thus, there are proposed various methods including a method of cutting a wire by holding the wire with a clamper and moving the wire upward after a capillary and the clamper are moved transversely such that a central axis of vertical movement of the capillary and the clamper is aligned with an end portion of bonding before holding the wire with the clamper and pulling the tail wire upward (see Japanese Patent No. 2723277, for example), and a method of moving a capillary in a transverse direction at the same time as an application of ultrasonic vibration in bonding (see Japanese Patent No. 3128718, for example). There are also proposed a method of opening a clamper to extend a tail wire, causing the capillary to vibrate at a natural frequency of the tail wire, and resonating the tail wire and cut the wire, thereby cutting the tail wire by the vibration (see Japanese Patent No. 2969953, for example), and a method of causing a capillary to perform a scrubbing operation when bonding while pressing the wire against a bonding surface using the capillary (see Japanese Patent No. 2530224, for example).
Depending on the bonding condition, the wire is possibly cut during the bonding and falls out from the capillary (fall-out of the wire). There is proposed a method of, when the fall-out of the wire occurs, causing a wire-holding clamper to close and a wire-cutting clamper to open, moving the wire-cutting clamper upward along with the capillary to a position higher than the wire-holding clamper, thereby inserting the wire into the capillary (see Japanese Examined Patent Application Publication No. H01-35500, for example).
However, according to the conventional bonding methods described in Japanese Patent No. 2723277 and Japanese Patent No. 2969953, when bonding the wire to the lead, it is required to adjust such as a pressing force of the capillary such that the cross-sectional area of the end portion of the wire bonded to the lead is smaller than that of other portions of the wire. If the pressing force of the capillary is too large, the wire is cut at the same time as the wire is bonded to the lead and the wire falls out from the capillary, resulting in the fall-out of the wire. If the pressing force of the capillary is too small, it is necessary to move the wire upward with a greater strength even with the method described in Japanese Patent No. 2723277 or Japanese Patent No. 2969953, and thus the wire is adversely deformed into an S shape. Moreover, according to the conventional methods described in Japanese Patent No. 3128718 and Japanese Patent No. 2530224, the problem of the fall-out of the wire can occur as the wire can be cut as in the case of the conventional techniques disclosed in Japanese Patent No. 2723277 or Japanese Patent No. 2969953, unless adjustment is made to prevent the wire from being cut when the capillary moves in a transverse direction. In other words, the conventional techniques described in Japanese Patents No. 2723277 to No. 2530224 pose a problem that the occurrence of the fall-out of the wire and the bend in the wire can be suppressed only when the pressing force of the capillary and such are constantly adjusted to be within a certain range. In addition, the conventional techniques described in Japanese Patents No. 2723277 to No. 2530224 poses another problem that there is a case in which a bonding strength between the wire and the lead cannot be ensured as the pressing force is required to be maintained within a certain range.
An object of the present invention is to suppress a fall-out and a bend in a wire in a further effective manner.